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Transcript of Meiosis Simplified
In Prophase 1, the homologous chromosomes have been replicated, and sister chromatids are joined at the centromere
The non-sister chromatids cross over: they overlap and exchange bits of non-sister chromatids in a very precise fashion (in this example, a small piece of the end of one blue chromatid is exchanged with the end of the red chromatid...it is like cutting and pasting them together)
The result are chromosomes that contain bits of chromsomes from non-sister chromatids
This does NOT happen in mitosis.
Similar to prophase of mitosis, the nuclear envelope is disappearing, the centrioles have duplicated and are starting to make spindle fibers and the chromosomes have condensed.
In Metaphase 1, homologous pairs of chromosomes line up at the equator of the cell
Spindle fibers from the chromatids connect to the kinetochore of the centromeres
No crossing over occurs during metaphase - it ONLY occurs during Prophase 1
In Anaphase 1, homologous pairs of chromsomes are separated while sister chromatids stay joined at the centromere
Compare this to mitosis where sister chromatids are separated during anaphase
End of Meiosis!
A the end of meiosis, you have 4 daughter cells
The cells are haploid (they have half as many chromosomes as regular somatic cells
These cells are only used for reproduction (sperm and eggs) - so having half the number of chromosomes is important (so the regular chromosome number is reestablished after fertilization
In Anaphase 2, sister chromatids are separated
Notice that 2 of the chromatids are a mosaic of blue and red chromosomes - these are the products of crossing over in Prophase 1, and they create an enormous variety of chromosomes combinations
Everyday I'm Shufflin'...
In Telophase 1, the sister chromatids are still joined
One round of cell division occurs producing 2 cells (they look diploid, but because they contain 2 copies of the same chromosome, they are technically haploid cells at this point)
The nuclei may briefly reform, but they disappear quickly again in Prophase 2
In Prophase 2, the nuclear envelope breaks down, the centrioles have duplicated and spindle fibers are starting to develop again (not shown)
NO crossing over occurs in Prophase 2!!
In the second metaphase, the sister chromatids in each cell are still joined at the centromere
The centrioles line these up at the middle of the cell using spindle fibers
In telophase 2, the sister chromatids have been completely separated, and a total of 4 daughter cells have been formed
Nuclear membranes are reforming, and chromsomes are starting to unwind